Technical Field
The present invention relates to an air-conditioning control apparatus that controls operation of an air-conditioning unit and is used in a central air-conditioning system, in which the central air-conditioning system conditions a plurality of rooms in a house by a single air-conditioning unit that performs at least a heating operation.
Related Art
The air-conditioning unit of a central air-conditioning system such as this includes a plurality (multiple stages) of air-conditioners. The operating ability of the air-conditioning unit can be changed by the operations of the air-conditioners being switched ON/OFF. Each air-conditioner of the air-conditioning unit performs an operation, such as heating, when supplied with a command signal from the air-conditioning control apparatus. The command signal is used to command the air-conditioner to perform the operation. When the supply of control signal is stopped, the air-conditioner stops the operation (refer to, for example, JP-A-2012-52769).
An alternating-current voltage (such as 24 V) is often used as the above-described command signal. The alternating-current voltage is generated by a power supply circuit that is provided in the air-conditioning unit. In this case, the air-conditioning unit outputs the alternating-current voltage to the air-conditioning control apparatus. Then, based on whether or not the alternating current voltage provided by the air-conditioning unit is to be supplied again to the air-conditioning unit, the air-conditioning control apparatus switches the operation of each air-conditioner so as to be performed or stopped (ON/OFF).
A latching relay is used to perform the above-described switching. The latching relay is used for the following reason. In other words, the contact of the latching relay is opened and closed by a drive current (excitation current) being supplied. The latching relay maintains the current state (open or closed) even when the supply of drive current is stopped. Therefore, once the contact is actuated, the drive current is not required to be sent to maintain the state of the contact. Conversely, in a non-latching relay, the drive current is required to be continuously sent to maintain the actuated state of the contact.
When the non-latching relay is used to perform the above-described switching, the drive current flows at all times to the excitation coils of a plurality of non-latching relays while the command to perform the operation is being issued. Therefore, the excitation coils generate heat. As a result, the temperature inside the housing of the air-conditioning control apparatus significantly increases. The air-conditioning control apparatus includes a temperature sensor within the housing. The temperature sensor is used to measures the room temperature. Therefore, when the temperature inside the housing significantly increases, the temperature sensor cannot accurately measure the room temperature.
Conversely, when the latching relay is used to perform the above-described switching, the drive current is not required to be continuously sent while the command to perform the operation is being issued. Therefore, the excitation coils generate little heat. As a result, the temperature inside the housing of the air-conditioning control apparatus does not significantly increase. The temperature sensor can accurately measure the room temperature.
However, when the latching relay is used to control the output of alternating-current voltage (command signal) to the air-conditioning unit, the following problem occurs. In other words, in all relays including the latching relay, an arc occurs when a contact is opened and closed while current is flowing. The arc may cause welding (sticking) of the contact. In addition, sticking of the contact may also occur as a result of degradation over time and the like.
When sticking of the contact occurs in this way, the state in which the command to perform the operation is being issued cannot be terminated. At this time, the air-conditioning unit may be performing the heating operation. In this case, the room temperature may increase to a temperature that is significantly higher (referred to, hereinafter, as an abnormally high temperature) than a temperature within a range that is normally considered suitable. In addition, the supply of power supply voltage to a control system, such as a microcomputer, that controls the opening and closing the latching relay may be stopped while the contact of the latching relay is closed. In this case as well, the state in which the contact is closed, or in other words, the state in which the command to perform the operation is being issued cannot be terminated. A problem occurs that is similar to that when sticking of the contact occurs.
Therefore, the air-conditioning control apparatus is configured to include a switch using a bimetal (referred to, hereinafter, as a bimetal switch). The bimetal switch is interposed on a power supply path between a voltage input terminal and the contact of the latching relay. The alternating-current voltage outputted from the air-conditioning unit is inputted into the voltage input terminal. The bimetal switch is configured by the bimetal and a contact. The bimetal is composed of two types of metal that differ in terms of thermal expansion and are bonded together. The bimetal switch is opened and closed by the bimetal expanding as a result of temperature change, thereby actuating the contact.
In a configuration such as this, even when the state in which the contact of the latching relay is closed cannot be terminated, the contact of the bimetal switch is turned OFF (opened) when the room temperature increases to the vicinity of a predetermined temperature. As a result, the supply of alternating-current voltage (command signal) to the air-conditioning unit is stopped. The heating operation is stopped. Therefore, a situation in which the room temperature increases to an abnormally high temperature can be prevented from occurring.
In a steady state, the bimetal switch self-heats depending on the current flowing to the bimetal. In addition, the current flowing to the bimetal changes depending on the number of latching relays that are in the ON state. Therefore, the amount of heat generated by the bimetal switch in the steady state changes depending on the number of air-conditioners that are performing the operation. As a result, the OFF-setting temperature of the bimetal switch changes (varies) depending on the number of air-conditioners that are performing the operation, or in other words, the variable state of the operating ability of the air-conditioning unit.
In light of such issues, it is difficult to set the temperature at which the operation of the air-conditioning unit is forcibly stopped, in the configuration in which the bimetal switch is used. For example, if the OFF-setting temperature of the bimetal switch is set to a lower temperature, the variation may cause a malfunction to occur in which air-conditioning is stopped regardless of the room temperature being within a normally expected temperature range (referred to, hereinafter, as a normal-range temperature). In addition, if the OFF-setting temperature of the bimetal switch is set to a higher temperature, the variation may cause a situation in which air-conditioning is not stopped even though the room temperature has reached an abnormally high temperature.